Such radiation detectors are known, for example, through French patent FR 2 605 166 in which a sensor formed from amorphous silicon photodiodes is associated with a radiation converter.
The structure and operation of such a radiation detector will be briefly recalled.
The photosensitive sensor is generally made from solid-state photosensitive elements arranged in a matrix. The photosensitive elements are made from semiconductor materials, usually from mono-crystalline silicon for sensors of the CCD or CMOS type, from poly-crystalline or amorphous silicon. A photosensitive element comprises at least one photodiode, one phototransistor or one photo-resistor. These elements are deposited onto a substrate, generally a glass plate.
These elements are not generally directly sensitive to very short wavelength radiation such as X-rays or gamma rays. This is the reason why the photosensitive sensor is associated with a radiation converter which comprises a layer of a scintillating substance. When it is excited by such radiation, this substance has the property of emitting radiation of a longer wavelength, for example visible or near-visible light, to which the sensor is sensitive. The light emitted by the radiation converter illuminates the photosensitive elements of the sensor which perform a photoelectric conversion and deliver electrical signals which are exploitable by appropriate circuits. The radiation converter will be referred to as the scintillator in the description that follows.
Certain scintillating substances of the alkaline halide or rare earth oxysulfide family are frequently employed for their good performances.
Among the alkaline halides, cesium iodide, doped with sodium or thallium according to whether an emission around 400 nanometers or around 550 nanometers, respectively, is desired, is known for its strong X-ray absorption and its excellent fluorescence efficiency. It takes the form of fine needles that are grown on a support. These needles are more or less perpendicular to this support and they partially confine the light emitted toward the sensor. Their fineness determines the resolution of the detector. Lanthanum and gadolinium oxysulfides are also widely employed for the same reasons.
However, some of these scintillating substances have the drawback of poor stability, being subject to a partial decomposition when they are exposed to water vapor, and their decomposition liberates chemical species that migrate either toward or away from the sensor. These species are very corrosive. Cesium iodide and lanthanum oxysulfide suffer notably from this drawback.
In relation to cesium iodide, its decomposition yields cesium hydroxide Cs+ OH− and free iodine 12 which can then combine with iodide ions to yield the complex radical I3−.
In relation to lanthanum oxysulfide, its decomposition yields hydrogen sulfide H2S that is chemically very aggressive.
Water vapor is extremely difficult to eliminate. It is always present in ambient air as well as in the bonding compound used in the detector assembly. The presence of water vapor in the bonding compound is due either to the ambient air or as a by-product of the polymerization if the latter results from the condensation of two chemical species, which is frequently the case.
One of the important aspects during the manufacture of these detectors will be to minimize the amount of water vapor initially present inside the detector, and in contact with the scintillator, and to avoid the diffusion of this water vapor into the sensor during its operation.
Radiation detectors include an entry window through which the X-rays pass upstream of the scintillator. In addition, the scintillating substance is generally deposited onto a metallic support, the support and the scintillating substance thus forming the scintillator. Furthermore, a known solution is to use the support as an entry window.
When the scintillating substance is deposited onto the entry window to form the scintillator which is then placed on the sensor, the entry window must withstand the thermal stresses of the deposition and processing of the scintillator without being damaged and must preferentially have a thermal expansion coefficient close to that of the scintillator and to that of the sensor, or more especially to that of its substrate. The window may also be chosen to have a low modulus of elasticity, which will tend to eliminate differential stresses between on the one hand the window and the scintillator and on the other hand the window and the sensor, or more particularly the sensor substrate. Thus, the risks of the scintillator cracking or of the sensor substrate breaking are avoided.
The condition of the surface of the entry window must additionally allow the growth, especially for cesium iodide, of the finest possible needles in the most uniform manner possible. The fineness of the needles is a factor in the quality of the detector resolution.
The supports are currently made of aluminum which has an excellent transparency to the radiation to be detected and good optical properties. After a conditioning treatment of the aluminum, a satisfactory surface condition for deposition of the scintillator can be obtained. Unfortunately, its thermal expansion coefficient is very different from that of the sensor. In order to avoid significant mechanical stresses at the interface between the two elements during thermal cycling, the use of a supple leak-tight seal, capable of absorbing without damage the deformations caused by this thermal cycling, is called for. This seal is necessarily supple in order to absorb the differences in thermal expansion between the scintillator support and the sensor during thermal cycling, so as to minimize stresses and the risks of fracturing. However, supple materials are generally permeable to water vapor, which results in an insufficient protection of the scintillator against this water vapor, and hence a reduced lifetime for the detector. It is desirable for such radiation detectors to have a lifetime comparable to the cost recovery time for radiological or other equipment onto which they are mounted, which is around 10 years.